Bug#1057505: grengine: FTBFS with default Java 21

[Vladimir Petko]

Source: grengine
Version: 3.0.2+dfsg-1
Severity: important
Tags: ftbfs
User: debian-java at lists.debian.org
Usertags: default-java21

Dear Maintainers,

The package grengine ftbfs with default Java 21.
The relevant part of the build log:
---------------
All input files are considered out-of-date for incremental task ':compileJava'.
Compiling with JDK Java compiler API.
/<<PKGBUILDDIR>>/src/main/java/ch/artecat/grengine/Grengine.java:145: warning: [this-escape] possible 'this' escape before subclass is fully initialized
        updateEngineIfSourcesLayersModified();
                                           ^
error: warnings found and -Werror specified
/<<PKGBUILDDIR>>/src/main/java/ch/artecat/grengine/except/ClassNameConflictException.java:38: warning: [serial] non-transient instance field of a serializable class declared with a non-serializable type
    private final Map<String,List<Code>> sameClassNamesInMultipleCodeLayersMap;
                                         ^
/<<PKGBUILDDIR>>/src/main/java/ch/artecat/grengine/except/ClassNameConflictException.java:39: warning: [serial] non-transient instance field of a serializable class declared with a non-serializable type
    private final Map<String,List<Code>> sameClassNamesInParentAndCodeLayersMap;
                                         ^
/<<PKGBUILDDIR>>/src/main/java/ch/artecat/grengine/except/CompileException.java:34: warning: [serial] non-transient instance field of a serializable class declared with a non-serializable type
    private final Sources sources;
                          ^
1 error
4 warnings
:compileJava FAILED
:compileJava (Thread[#29,Daemon worker,5,main]) completed. Took 2.205 secs.

FAILURE: Build failed with an exception.
---------------


-- System Information:
Debian Release: trixie/sid
  APT prefers mantic-updates
  APT policy: (500, 'mantic-updates'), (500, 'mantic-security'), (500, 'mantic')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 6.5.0-13-generic (SMP w/32 CPU threads; PREEMPT)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_OOT_MODULE
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled